best way to visit seville & granada, i must start & end in barcelona

'recommended travel plan via rail?' - not to fly into and out of Barcelona if interested in Seville and Granada :-) It is not less than 10 hours Barcelona to Granada, not less than 5 hours Barcelona to Seville. Yes, fine Seville to Granada or reverse at 3 hours (or bus). Take advantage of up to 70% advance purchase discounting if you have a fixed itinerary - explanation here - http://www.seat61.com/Spain-trains.htm#How_to_use_www.renfe.com Vueling have low cost flights which might work for you.

Do you also want some time in Madrid? Then you could take the AVE train Barcelona to Sevilla, 5 1/2 hours, train to Granada, slow train back to Madrid, about 4 1/2 hours, spend a few days there, fast train back to Barcelona, 2 1/2 hours. Buy your tickets well in advance (62 days) to get 60% off the AVe trains and a good price on the Altaria from Granada to Madrid. Some people will say to fly from Granada back to Barcelona. Keep in mind that rail travel is much cleaner. Spain has invested a lot on their high-speed rail network to make travel easier and greener. So I advocate for using their trains whenever possible.

You can go between Barcelona and Sevilla by highspeed rail (AVE) in about 5.5 hours. There are two direct trains a day but you can also go to Madrid and switch trains there. That would increase your travel time to 6-7 hours but give you more options. With advance purchase, the cost can be very reasonable. Granada and Barcelona are not well connected by train. I have heard there is an overnight train but I don't see it still listed on RENFE. There are ways to connect in Madrid, but mostly it's an over 8 hour trip. Granada and Sevilla are well connected by train and even better by bus. It's about a 3 hour trip. But is cheaper and in many ways easier. Unless you want to make stops in between, the fastest/best way between Granada/Sevilla and Barcelona is to fly. You can get pretty inexpensive flights on various airlines - check around. Some severely limit baggage and will charge extra, so plan for that based on their policy.

Last time I did Seville to Barcelona I flew, but that was before they opened the Madrid-Barcelona AVE in 2009. Seville to Barelona can be done with the AVE train now (5.5 hours) but unless you purchase early a discounted ticket the base price is stiff at Euro 145 ($200) one way. Vueling charges Euro 45 to 75 (depending on departure time) for a flight from either Granada or Sevilla to BCN. Flight time is about 1h 15 min, but of course you need to add the check-in/security time and the transfer from airport to hotel (airports are about 20 min. from all the above cities) therefore door to door you'll looking at about 4 hr travel, maybe a 2 hr savings over train in total. If you can secure early a discounted ticket on the AVE train, the prices are comparable to flying. In your situation I would do: BARCELONA-SEVILLA (either AVE train or Vueling flight, depending on prices available and/or whether you prefer a train or a plane). SEVILLA-GRANADA (bus is a little cheaper than train and travel time is the same, 3 hours) GRANADA-BARCELONA (Vueling flight, since by train is way too long with no high speed to Granada) If you have time I would consider renting a car for the trip between Sevilla and Granada. That will give you a chance to roam around Andalucia's countryside and visit the small 'pueblos blancos' (Ronda, Arcos, Grazalema) or even Nerja, on the coast. You can return your car once you hit Granada, since you won't need your car there. You could do also the exact reverse loop.

I would book a flight with a low cost carrier (e.g. Vueling) to get from Barcelona to Granada. From there I would train to Seville and train back to Barcelona. Depending on the time of year, I'd plan some extra time to get my sea legs (adjust to the time) either in Barcelona or Granada. If it's August, I'd spend the extra time in Barcelona where it will be nice, but too hot in Granada. If it's shoulder season, I'd spend the time in Granada where it will be nice and could be chilly in Barcelona. Cordoba is right on your way. If you have the time, a half day there is nice to see the amazing Mesguita and wander that quarter. Madrid is on the way too. You have to change at Attocha station from your arriving train from Seville/Cordoba to your departing train to Barcelona. The Prado museum is only a few blocks from Attocha and IMO the single best sight in Madrid. Why not plan some time to visit the museum, and maybe the gardens next door, before heading to Barcelona?